Ferrous metal articles having a coating of metals that are dissimilar to the iron in the substrate on their surfaces have found a variety of industrial applications. The dissimilar metal coatings are typically comprised of aluminum either alone or aluminum in combination with other metals, such as zinc. This dissimilar metal coating provides corrosion protection to the ferrous metal substrate, but is itself subject to corrosion over time. Because of the dissimilar metal coating's tendency toward corrosion and environmental degradation, it is beneficial to provide the exposed surfaces of these metal articles with a secondary corrosion-resistant and protective coating. Such secondary coatings should resist abrasion so that the secondary and dissimilar metal coatings remain intact during use, where the metal article may be subjected to repeated contact with other surfaces, particulate matter and the like. Heat resistance is also a very desirable feature of a secondary protective coating. Where the appearance of the coated ferrous metal article is considered important, the secondary protective coating applied thereto should additionally be uniform and decorative.
In order to provide an effective and permanent protective coating on aluminum and its alloys, such metals have been anodized in a variety of electrolyte solutions, such as sulfuric acid, oxalic acid and chromic acid, which produce an alumina coating on the substrate. While anodization of aluminum and its alloys is capable of forming a more effective coating than painting or enameling, the resulting coated metals have still not been entirely satisfactory for their intended uses. The coatings frequently lack one or more of the desired degree of flexibility, hardness, smoothness, durability, adherence, heat resistance, resistance to acid and alkali attack, corrosion resistance, and/or imperviousness required to meet the most demanding needs of industry.
Anodization of ferrous metal substrates coated with an aluminum or aluminum alloy according to processes of the prior art results in an aluminum oxide coating that is brittle and requires subsequent sealing to provide a significant increase in corrosion protection. It is taught in the prior art that only certain metals, such as aluminum, magnesium, titanium and zinc, can be successfully anodized. It is also taught that electrically non-conductive substances, such as plastic, refractory materials and the like cannot be anodized.
Thus, there is still considerable need to develop alternative coating processes for non-conductive articles and ferrous metal articles having an aluminum or aluminum alloy metal coating which do not have any of the aforementioned shortcomings and yet still furnish corrosion-, heat- and abrasion-resistant protective coatings of high quality and pleasing appearance.
It will often be desirable to provide an anodized coating that not only protects the metal surface from corrosion but also provides a decorative white finish so that the application of a further coating of white paint or the like can be avoided. Few anodization methods are known in the art to be capable of forming a white-colored decorative finish with high hiding power on aluminum-coated ferrous metal substrates, for example.